I have previously blogged about that excellent tool Notepad++. Recently during discussion with a colleague I discovered that Notepad++ can be extended by means of a plug-in to be a Hexadecimal editor.
For more details see http://www.mydigitallife.info/use-notepad-as-hex-editor-with-plugin-download-free-winhex-alternative/
You can download Notepadd++ from http://sourceforge.net/projects/notepad-plus/ and plug-ins from http://sourceforge.net/projects/npp-plugins/
The licence is GPL, however this utility is actually freely usable for commercial use - see: